This invention relates to a polycrystalline composite compact of cubic boron nitride including titanium carbonitride and cobalt aluminide as a supporting phase, which is particularly useful as a tool for machining ferrous metals. In the process for making the compact a mixture of cubic boron nitride (CBN) crystals, titanium carbonitride (TiCN) and Co.sub.2 Al.sub.9 are treated at elevated temperature in ammonia to reduce oxygen and carbon and increase nitrogen. The mixture is then subjected to high temperatures and pressures where CBN is thermodynamically stable for forming polycrystalline cubic boron nitride.
The high pressure form of boron nitride, known as cubic boron nitride, is surpassed only by diamond in hardness and has a variety of uses as cutting tools and the like. Polycrystalline CBN is useful, for example, for high speed machining of ferrous metals where reaction of diamond with iron is a problem. Cutting tools are in the marketplace or are described in prior art patents employing cubic boron nitride and a binder phase of a refractory material such as titanium nitride, titanium carbide, titanium carbonitride and solid solutions thereof. Such compacts have excellent abrasive wear resistance, thermal stability, thermal conductivity, impact resistance and coefficient of friction for use as a cutting tool.
It is always desirable, however, to improve the quality, reliability, properties, lifetime and sharpness of materials employed for cutting tools.
U.S. Pat. No. 4,334,928 describes sintered compacts of CBN for use in cutting tools employing as a binder a variety of materials including carbides, nitrides, carbonitrides, borides and silicides of group IVa, Va and VIa group transition metals. According to this description, such materials in the binder phase form a continuous matrix with high pressure boron nitride (i.e., CBN) interspersed within the matrix. The principal material disclosed comprises titanium nitride as a nonstoichiometric material with an excess of titanium. Other exemplary materials include nonstoichiometric titanium carbide and titanium carbonitride. Some of the titanium compounds also include aluminum, tantalum, molybdenum or tungsten.
U.S. Pat. No. 5,271,749 discribes a sintered compact of CBN for use in cutting tools employing Co.sub.2 Al.sub.9 and an aluminum containing material such as aluminum, aluminum nitride or aluminum diboride as sintering adjuvants. The process employs a substantial portion of hexagonal boron nitride as well as cubic boron nitride as a starting material. The preferred amount of adjuvant materials is from 2 to 10% by weight. Preferably, aluminum nitride is present from 2 to 7% by weight and Co.sub.2 Al.sub.9 is present as from 1 to 4% by weight.
The blended ingredients are heat treated in a nonoxidizing atmosphere which may include ammonia at a temperature in the range of from 600.degree. to 950.degree. C. The mixture is placed on a cemtented tungsten carbide substrate and pressed at high temperature and pressure for forming a polycrystalline CBN layer on the carbide substrate. At the sintering temperature molten cobalt phase from the carbide substrate infiltrates into the mixture.
The U.S. Pat. No. 5,271,749 also describes an embodiment having a portion of titanium carbonitride (TiCN) comprising up to about 40% by weight of the total mixture. Other materials may be included such as carbide, nitride or carbonitride containing material of a group IVb, Vb or VIb transition metal. For example, a small amount of tungsten carbide may be included in the composition
It is found in practice of this invention that a composite material having a major proportion of polycrystalline cubic boron nitride with a minor proportion of a supporting phase formed from titanium carbonitride or the like, and cobalt aluminide performs appreciably better in machining ferrous materials and workpieces which are difficult to machine because of hard materials or gaps in the cutting path which impact upon the cutting tool.